Postfix relayhost trouble

Hi, I'm in charge of a postfix 2.5 server called club.school.edu at a university (school.edu). It is set to accept emails of the form [email protected] and redirect them to student's [email protected] via a relayhost called emailrelay.school.edu. Every time someone sends an email to [email protected] the email correctly arrives at [email protected], but the sender gets an additional email from the school.edu email server that says:

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Delivery has failed to these recipients or distribution lists:

[email protected]
The recipient's e-mail address was not found in the recipient's e-mail system. Microsoft Exchange will not try to redeliver this message for you. Please check the e-mail address and try resending this message, or provide the following diagnostic text to your system administrator.
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The mail.log entry on the club.school.edu server has the following
entry for the email:

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Delivery has failed to these recipients or distribution lists:

[email protected]
The recipient's e-mail address was not found in the recipient's e-mail system. Microsoft Exchange will not try to redeliver this message for you. Please check the e-mail address and try resending this message, or provide the following diagnostic text to your system administrator.
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